Process for the recovery of spent acid from a Mathieson or Solvay type chlorine dioxide generator and more particularly a process for the recovery of salt cake and reusable sulfuric acid from the effluent of a chlorine dioxide generator.
Those skilled in the art are aware that chlorine dioxide is used in the bleaching of cellulosic materials such as wood pulp produced by the Kraft or sulfite processes. It is also well known in the pulp and paper industry that the Hooker SVP and Erco R-3 systems offer an approach to the recovery of effluent from the chlorine dioxide generator by operating the generator under chemical conditions which favor precipitation of salt cake. The resystems have the disadvantage of generating significant quantities of chlorine gas which is dissolved in the chlorine dioxide and is harmful and damaging to the pulp.
On the other hand, the Mathieson or Solvay process does not produce chlorine gas but does yield an acid effluent which is difficult to dispose of since it cannot be dumped and to date, at least commercially, the acid effluent has not been successfully treated. Accordingly, the Mathieson or Solvay process for generation of chlorine dioxide is not used to any significant extent in the United States and Canada because of the environmental protection laws regulating the dumping of its effluent. As is also well known, recycling the C10.sub.2 generator effluent with the black liquor upsets chemical balances and values so that the Mathieson or Solvay system has fallen out of general use and favor worldwide.
It has been recognized in the prior art that in the evaporation and crystallization of the residual acid it is not only difficult but expensive to remove the sodium sulfate. However, while the industry is aware of the general chemistry involved, it has not developed a system for the purpose of disposing of the acid effluent in a closed system.
Among the prior art references known to applicants, only three would appear to be of interest. U.S. Pat. No. 4,329,199 is concerned with all of the environmental pollution substances for a pulp mill including black liquor, effluent from the bleaching operation, flue gases, condensate from the digester, and the like. This particular reference, however, does not direct itself to the process steps of this invention but simply mentions evaporation/crystallization steps and no more. The reference is considered to be of interest only for its recognition of the general chemistry.
U.S. Pat. No. 3,789,108 owned by Erco is directed to the production of chlorine dioxide. In this particular patent, a three-stage reaction zone for the production of chlorine dioxide involves the reacting of an alkali metal chlorate with alkali metal chloride and sulfuric acid to which in each of the generators air is introduced for removing chlorine. While the method of the Erco patent involves a three-stage operation for the production of sodium sulfate, its purpose is not to recover the acid effluent but to generate chlorine dioxide in each of the three stages. Accordingly, it adds sulfate ion-containing acidic material to all three of the stages.
Finally, U.S. Pat. No. 3,341,288 assigned to Hooker Chemical Corporation also is directed to the production of chlorine dioxide but does not concern itself with the acid effluent resulting from an Olin-Mathieson or Solvay type chlorine dioxide generating process.
Applicants are not aware of any other references at the time of filing this application that have any bearing on the steps of the instant application for the recovery of spent acid effluent in the form of salt cake and sulfuric acid.